1. Field of the Invention
The present invention relates generally to color signal matrix circuits and, more particularly, relates to an improved matrix circuit for producing two color difference signals from three primary color signals for use in a signal processing circuit of a color video camera.
2. Description of the Background
In a signal processing circuit used in a color video camera that produces a color video signal, three primary color signals including red, green, and blue, represented by R, G, and B, respectively, are produced based on a image pickup output signal obtained from the image pickup device of the camera. Aluminance signal, represented by Y, and two color difference signals, represented by (R-Y) and (B-Y), respectively, are then produced from the red, green, and blue primary color signals (R, G, and B) in order to form the desired color video signal. To accomplish such signal processing in a color video camera it has been recently proposed to use digital circuits, and one previously proposed digital circuit for use as a color signal matrix circuit for producing color difference signals from three primary color signals (R, G, B) is shown in FIG. 1.
In the circuit of FIG. 1, red, green, and blue primary color signals (R, G, and B) are each produced in digital form based on an image pickup output signal obtained from an image pickup device (not shown in FIG. 1) and supplied to input terminals 11, 12, and 13, respectively. The red primary color signal R at input terminal 11 is adjusted in level by a fixed level factor of 0.70 by a level adjuster 14 and is then supplied to a subtractor 19. The blue primary color signal B at input terminal 13 is also adjusted in level by a fixed level factor of 0.11 by a level adjuster 16 and is also supplied to subtractor 19. In subtractor 19 the blue primary color signal B obtained from level adjuster 16 is subtracted from the red primary color signal R obtained from the level adjuster 14 and a difference signal is produced and fed to a subtractor 21. The green primary color signal G at input terminal 12 is adjusted in level by a fixed level factor of 0.59 by a level adjuster 15 and then supplied to subtractor 21. In subtractor 21, the green primary color signal G obtained from level adjuster 15 is subtracted from the difference output obtained from subtractor 19 to produce an intermediate color difference signal (R-Y)' at the output of subtractor 21.
The red primary color signal R from input terminal 11 is also adjusted in level by a fixed level factor of 0.30 by a level adjuster 17 and then supplied to a subtractor 20. The blue primary color signal B from input terminal 13 is also adjusted in level by a level factor of 0.89 by a level adjuster 18 and then fed to subtractor 20. In subtractor 20, the level-adjusted (0.30) red primary color signal R obtained from level adjuster 17 is subtracted from the level-adjusted (0.89) blue primary color signal B obtained from level adjuster 18 to produce a difference output supplied to a subtractor 22. Further, the level-adjusted (0.59) green primary color signal G obtained from level adjuster 15 is also supplied to subtractor 22. In subtractor 22 the level-adjusted (0.59) green primary color signal G obtained from level adjuster 15 is subtracted from the difference output from subtractor 20 to produce an intermediate color difference signal (B-Y)' at the output of subtractor 22.
Fixed value level adjusters, such as elements 14-18, for digital signals are formed based on bit-shift type adders. Thus, the amount of level adjustment is fixed in each individual digital adder.
The color difference signal (R-Y)' obtained from subtractor 21 is supplied directly to an adder 23 and is also fed through a variable level adjuster 26, by which the color difference signal (R-Y)' is adjusted to have a relatively small level, to an adder 24. The color difference signal (B-Y)' obtained from subtractor 22 is supplied directly to adder 24 and is also fed through a variable level adjuster 25, by which the color difference signal (B-Y)' is adjusted to have a relatively small level, to adder 23. Consequently, the color difference signal (B-Y)' having a relatively small level is added in adder 23 to the color difference signal (R-Y)' derived from subtractor 21 to produce a summed output fed through a variable level adjuster 27 to an output terminal 29 as the desired color difference signal (R-Y). Further, the color difference signal (R-Y)' having the relatively small level is added in adder 24 to the color difference signal (B-Y)' derived from subtractor 22 to produce a summed output fed through a variable level adjuster 28 to an output terminal 30 as a color difference signal (B-Y).
Variable level adjusters, such as elements 25 and 26, for digital signals are formed based on digital multipliers. Thus, the amount of level adjustment is controlled by varying the multiplication factor in the digital multiplier. The purpose of these variable level adjusters is to compensate for nonuniformity or fluctuations in the values of each circuit element, that is, to compensate component value deviations within the permitted tolerance range. Thus, the amount of level adjustment provided by level adjusters 25,26 is quite small, on the order of 0.1 or 0.2, for example, and may be thought of as comprising trim adjustments. These adjustments are typically made at the manufacturing site of the video camera.
Each color difference signal (R-Y) and (B-Y) which is formed at adders 23 and 24, respectively, has been processed to correct imbalances in level among the red, green, and blue primary color signals (R, G, and B), which result from differences in the spectral-response characteristics among the image pickup elements for the red, green, and blue primary colors in the image pickup device of the color video camera and also has been compensated for circuit element tolerances.
FIG. 2 shows another previously proposed color signal matrix circuit for producing color difference signals from three primary color signals (R,B,and G). In FIG. 2, red, green, and blue primary color signals (R, G, and B) are each produced in digital form based on an image pickup signal output from an image pickup device (not shown in FIG. 2) and supplied to input terminals 31, 32, and 33, respectively. The red primary color signal R at input terminal 31 and the green primary color signal G at input terminal 32 are supplied to a subtractor 34, in which the green primary color signal G is subtracted from the red primary color signal R, and a difference output represented by (R-G) is obtained. Further, the blue primary color signal B at input terminal 33 and the green primary color signal G at input terminal 32 are supplied to a subtractor 35, in which the green primary color signal G is subtracted from the blue primary color signal, B and a difference output represented by (B-G) is obtained.
The subtraction output (R-G) of subtractor 34 is adjusted in level to a level factor of 0.70 by a level adjuster 36 and then supplied to a subtractor 40, and the subtraction output (B-G) is also adjusted in level by a level factor of 0.11 in a level adjuster 37 and then supplied to subtractor 40. In subtractor 40 the level-adjusted difference output (B-G) derived from level adjuster 37 is subtracted from the level-adjusted difference output (R-G) from level adjuster 36 to produce an intermediate color difference signal (R-Y)'. Further, the subtraction output (R-G) from subtractor 34 is also adjusted in level by a level factor of 0.30 in a level adjuster 38 and then fed to a subtractor 41, and the subtraction output (B-G) from subtractor 35 is also adjusted in level by a level factor of 0.89 in a level adjuster 39 and then supplied to subtractor 41. In subtractor 41, the level-adjusted difference output (R-G) derived from level adjuster 38 is subtracted from the level-adjusted difference output (B-G) derived from level adjuster 39 to produce an intermediate color difference signal (B-Y)'.
The color difference signal (R-Y)' obtained from subtractor 40 is fed directly to an adder 42 and also through a variable level adjuster 45, by which the color difference signal (R-Y)' is adjusted to a relatively small level, to an adder 43. The color difference signal (B-Y)' obtained from subtractor 41 is fed directly to adder 43 and through a variable level adjuster 44, by which the color difference signal (B-Y)' is adjusted to a relatively small level, to adder 42. Consequently, a color difference signal (B-Y)' having a relatively small level is added to the color difference signal (R-Y)' derived from subtractor 40 to produce an output at adder 42 that is fed through a final variable level adjuster 46 to an output terminal 48 as the desired color difference signal (R-Y). Similarly, the color difference signal (R-Y)' having the relatively small level is added to the color difference signal (B-Y)' derived from subtractor 41 to produce an output at adder 43 that is fed through a final variable level adjuster 47 to an output terminal 49 as the desired color difference signal (B-Y).
In the circuit of FIG. 2, each of the color difference signals (R-Y) and (B-Y) that are formed at adders 42 and 43, respectively, has been processed to correct level imbalances among the red, green, and blue primary color signals (R, G, and B) resulting from differences in spectral-response characteristics among the respective image pickup elements for the red, green and blue primary colors in the image pickup device.
In each of these previously proposed color signal matrix circuits of FIGS. 1 and 2, however, there is an inherent disadvantage that the configuration of the entire circuit becomes complicated and must be constructed on a relatively large scale, because generally a separate adder or subtractor is required for each adding or subtracting operation of two input signals in a digital circuit and, therefore, the number of adder and subtract circuits is inevitably increased in the case where adding and subtracting operations for three input signals are required. In addition, digital bit-shift adders and digital multipliers take up relatively large volumes in these digital circuits and the requirement to use large numbers of these elements increases the overall size of the circuits. There is also the disadvantage that the color matrix circuit is not easily adaptable for incorporation into a large scale integrated circuit (LSI).